Imagine a chain of volcanoes extending over 640 kilometers, buried six kilometers under our feet, silent for hundreds of millions of years. This is exactly what geologists have just discovered under southern China: a real fossilized volcanic empire which testifies to an era when our planet underwent titanic upheavals. This extraordinary discovery may well rewrite our understanding of the climate history of the earth and reveal how ancient geological cataclysms have shaped the world we know today.
Detection worthy of a science fiction film
The story of this discovery begins with a colossal technical challenge: how to observe what hides miles below the earth’s surface? The team led by Zhidong Gu of Petrochina and Junyong Li from the University of Nanjing has taken up this challenge by deploying fascinating technology: airport magnetic sensors capable of “radiographing” the earth’s crust.
Their method is based on an elegant principle: each type of rock contains different magnetic minerals, creating a unique magnetic signature. By flying over the Sichuan basin with their hypersensitive detectors, scientists have gradually reconstructed an underground map revealing the unthinkable.
Under several kilometers of sedimentary rocks accumulated over the eons, they identified a rocky strip rich in iron with striking geometry: 700 kilometers in length by 50 kilometers in width, snapping from northeast to southwest. This geological formation exhibited an abnormally intense magnetic field, betraying its old volcanic nature.
Voyage to the heart of geological time
To understand this discovery, it is necessary to go back in time until a time when the earth was unrecognizable. 800 million years ago, at the beginning of the neoproterozoic, our planet housed a supercontinent called Rodinia. Southern China then occupied a strategic position at the northwest limit of this gigantic continental mass.
But Rodinia was sentenced to bursting. The titanic tectonic forces which animate the interior of our planet caused its fragmentation, violently separating what would become the Yangtze block from the rest of the supercontinent. This continental break was not a peaceful phenomenon: it was accompanied by cataclysmic collisions between tectonic plates.
When the Yangtze block collided with the Oceanic plate of China, a geological process of incredible violence has started. The more dense oceanic crust plunged under the continental crust into a subduction phenomenon that would transform the landscape for hundreds of millions of years.
Reconstitution of Rodinia placing southern China between Australia-Antarctic and Laurentia. Credit: Chao Liu/Earthbyte.
The birth of an empire of fire
This underground tectonic ballet has triggered a cascade of spectacular volcanic phenomena. When the oceanic crust sinks into the terrestrial depths, it warms and releases the water it contains. This water, going up, generates magma which finds its path to the surface, fueling volcanoes arranged in arc above the subduction zone.
But the volcanic arc discovered under Southern China defies classic models. Unlike the usual volcanic chains which form relatively narrow belts along the coasts-such as North American waterfalls-this formation covers nearly 900 kilometers inside the land.
The researchers analyzed rocky samples taken during deep boreholes, confirming their magmatic origin and their age: between 770 and 820 million years. These rocks bear the characteristic chemical signature of arc volcanoes, permanently validating the hypothesis of an exceptional volcanic system.
An exceptional geological mechanism
To explain this unusual configuration, scientists evoke a rare phenomenon called “flat plate subduction”. In this scenario, the ocean plate does not plunge directly into the terrestrial depths but slides horizontally under the continent over hundreds of kilometers before sinking permanently.
This particular process generates two distinct volcanic fronts: a first near the coast, where the plate begins to slide under the continent, and a second much more inside the land, where it ends up plunging into the earthly coat. A similar phenomenon is shaping the Andes Cordillera today, where the Nazca plate generates several parallel mountainous chains.
Major climatic implications
This discovery goes beyond the purely geological framework to touch the great climatic balances of our planet. Geochemical statements indicate that the overall carbon cycle underwent major disruptions during this period, between 720 million and 1 billion years.
Volcanoes play a crucial role in this cycle: they reject carbon dioxide in the atmosphere, while the erosion of the mountains they create consumes this same gas. The unexpected magnitude of ancient Chinese volcanism suggests that its impact on the earth’s climate may have been considerably underestimated.
This revelation opens up new perspectives to understand the climatic instabilities of neoproterozoic and their influence on the evolution of life on earth, at a crucial era which preceded the explosion of the biodiversity of the Cambrian.
